Update LuaJIT

[ayende]

Just FYI, certain scripts can cause LuaJIT's string hashing to go into O(N^2) mode

This was fixed here: LuaJIT/LuaJIT#555

But the LuaJIT version bundled with wrk2 is a lot older.

Sample scenario:

local cnt = 0
local prefix = nil -- change to '' for perf boost 

request = function ()
    c = c + 1
    local f = string.format('aaaa-%s-%6d-zzzz', prefix, c)
     return wrk.format(nil, '/run?id=' .. f)
end

This will hit the issue and cause wrk2 to slowly become slower and slower over time.